Some young children develop an allergy to milk, with symptoms ranging from swelling of the eyes and face to nausea and difficulty breathing. Other children, as they mature, develop intolerance to lactose, the sugar found in milk, making it difficult for them to digest it.

Most children with a milk allergy will outgrow it by the time they turn 3, according to the American Academy of Allergy, Asthma and Immunology (AAAAI). Most of the rest will outgrow it by the time they head to school. Children who develop milk intolerance will not outgrow it.

It is important to understand the difference between food allergy and food intolerance. A food allergy, or hypersensitivity, is an abnormal response to a food that is triggered by the immune system, the AAAAI says. The immune system is not responsible for the symptoms of food intolerance, even though the symptoms may resemble those of a food allergy.

Missing enzyme

Intolerance to a food may mean the body does not have an enzyme necessary to digest a particular part of the food. Food intolerance is not a life-threatening condition. With milk, it is the lactose that many people cannot digest.

"When there is not enough lactase [an enzyme] to digest the amount of lactose consumed, the results may be very distressing -- stomach cramps, bloating, gas, and/or diarrhea," says Stanley Goldstein, M.D., a Long Island immunologist.

Children and adults who develop milk intolerance fail to produce enough lactase. But over-the-counter pills can provide the missing enzyme. Milk intolerance is considered a mild, if annoying, problem.

Allergies and the immune system

A food allergy is an immune system response to a food that the body mistakenly believes is harmful. Once the immune system decides that a particular food is harmful, it creates specific antibodies to it. In milk -- and all food allergies -- the culprits are proteins.

The next time the person eats that food (and those proteins), the immune system releases massive amounts of chemicals, including histamine, to protect the body. These chemicals trigger a cascade of allergic symptoms that can affect the respiratory system, gastrointestinal tract, skin, or cardiovascular system, the AAAAI says. Symptoms may include a tingling sensation in the mouth, swelling of the tongue and throat, difficulty breathing, hives, vomiting, abdominal cramps, diarrhea, a drop in blood pressure, and loss of consciousness. Symptoms typically appear within minutes to two hours after the person has eaten the food to which he or she is allergic and can end in death, especially if not treated promptly.

Fortunately, milk allergies rarely cause such a severe reaction, Dr. Goldstein says. And, he says, "one important difference between milk allergy and lactose intolerance is that lactose intolerance is not a life-threatening condition. In fact, people with lactose intolerance often can consume small amounts of milk without experiencing any symptoms."

The majority of children who are allergic to the proteins in cow's milk are also allergic to goat's and sheep's milk. Symptoms of milk allergy include a red, bumpy skin rash called hives; eczema; redness and swelling around the mouth; stomach cramps; loose stools, possibly with blood; nausea or vomiting; runny nose; congestion; watery eyes; sneezing; and coughing. Most children with a milk allergy have more respiratory and ear infections than children without the allergy.

Very few children with a milk allergy need to avoid milk. Most children lose their milk allergy by age 3 and almost nearly all by ages 5 to 6.

Two types of hypoallergenic formulas are approved by the U.S. Food and Drug Administration for infants with a milk allergy. One is made from cow’s milk in which the proteins have been broken down into small particles; the other is made from amino acids, the building blocks of proteins.

Prevention through breast-feeding

One way to help prevent a milk allergy in children is to breast-feed. Breast-feeding reduces the risk of an allergic reaction to cow's milk, although it doesn't eliminate it completely, the AAAAI says. The protein in cow's milk consumed by the mother can find its way into the breast milk.

The American Dietetic Association (ADA) recommends that babies be breast-fed exclusively for four to six months and then breast-fed while solid foods are introduced, for a total of at least 12 months.

Infants who are breast-fed for that length of time seem to have a better chance of avoiding food allergies, the ADA says.